- A
The new SFP is a counterfeit Cisco transceiver that cannot establish a stable link.
Why wrong: A counterfeit SFP may cause operational issues such as link flapping or being placed in an error-disabled state due to Cisco's transceiver validation, but it would not typically cause a drastically low optical receive power reading—the power measurement reflects actual light received.
- B
The SFP is not fully seated in the switch port, causing an intermittent optical connection.
Why wrong: If the SFP were not fully inserted, the fiber would likely not couple light into the receiver at all, resulting in no Rx power measurement (e.g., '-Inf' or '--'), not a specific –30 dBm reading that indicates some light is reaching the photodiode.
- C
Excessive attenuation due to a dirty or damaged fiber connector is preventing the link from coming up.
A –30 dBm Rx power with a transmit level of –3 dBm over a 2 km single-mode span represents a 27 dB loss, far exceeding the expected 0.5–1 dB. Such high loss is typical of contaminated end faces, poor mating, or a tight bend, and it pushes the signal below the receiver sensitivity threshold (around –25 dBm), causing the link to stay down.
- D
The single-mode fiber distance exceeds the 10 km maximum for 1000BASE-LX, leading to severe signal dispersion.
Why wrong: The fiber run is only 2 km, well within the LX specification. Even at 10 km, single-mode fiber would introduce only about 5 dB of attenuation (0.5 dB/km), resulting in a receive power of roughly –8 dBm, not –30 dBm. Dispersion does not cause a power-level drop; it distorts the signal shape.
CCNA Network Infrastructure and Connectivity Practice Question
This 200-301 practice question tests your understanding of network infrastructure and connectivity. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A network engineer replaces a failed 1000BASE-LX SFP on a core switch with a new transceiver of the same type. After connecting the single-mode fiber, the link remains down and a 'show interfaces gig1/0/49 transceiver' reveals an Rx power of –30 dBm, while the far-end SFP is transmitting at –3 dBm over a 2 km span. The fiber patch cord shows no visible damage.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Excessive attenuation due to a dirty or damaged fiber connector is preventing the link from coming up.
Option C is correct because the measured Rx power of –30 dBm is far below the receive sensitivity threshold for 1000BASE-LX (typically –19 to –22 dBm), even though the transmitter is outputting a healthy –3 dBm over only 2 km. This indicates excessive loss in the optical path, most commonly caused by a dirty or damaged fiber connector. Cleaning the connector ends with an appropriate fiber cleaning tool and inspecting with a microscope would likely resolve the issue.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The new SFP is a counterfeit Cisco transceiver that cannot establish a stable link.
Why it's wrong here
A counterfeit SFP may cause operational issues such as link flapping or being placed in an error-disabled state due to Cisco's transceiver validation, but it would not typically cause a drastically low optical receive power reading—the power measurement reflects actual light received.
- ✗
The SFP is not fully seated in the switch port, causing an intermittent optical connection.
Why it's wrong here
If the SFP were not fully inserted, the fiber would likely not couple light into the receiver at all, resulting in no Rx power measurement (e.g., '-Inf' or '--'), not a specific –30 dBm reading that indicates some light is reaching the photodiode.
- ✓
Excessive attenuation due to a dirty or damaged fiber connector is preventing the link from coming up.
Why this is correct
A –30 dBm Rx power with a transmit level of –3 dBm over a 2 km single-mode span represents a 27 dB loss, far exceeding the expected 0.5–1 dB. Such high loss is typical of contaminated end faces, poor mating, or a tight bend, and it pushes the signal below the receiver sensitivity threshold (around –25 dBm), causing the link to stay down.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The single-mode fiber distance exceeds the 10 km maximum for 1000BASE-LX, leading to severe signal dispersion.
Why it's wrong here
The fiber run is only 2 km, well within the LX specification. Even at 10 km, single-mode fiber would introduce only about 5 dB of attenuation (0.5 dB/km), resulting in a receive power of roughly –8 dBm, not –30 dBm. Dispersion does not cause a power-level drop; it distorts the signal shape.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Excessive attenuation due to a dirty or damaged fiber connector is preventing the link from coming up.Correct answer▾
Why this is correct
A –30 dBm Rx power with a transmit level of –3 dBm over a 2 km single-mode span represents a 27 dB loss, far exceeding the expected 0.5–1 dB. Such high loss is typical of contaminated end faces, poor mating, or a tight bend, and it pushes the signal below the receiver sensitivity threshold (around –25 dBm), causing the link to stay down.
✗The new SFP is a counterfeit Cisco transceiver that cannot establish a stable link.Wrong answer — click to see why▾
Why this is wrong here
Low Rx power points to a physical signal issue, not a counterfeit detection problem.
✗The SFP is not fully seated in the switch port, causing an intermittent optical connection.Wrong answer — click to see why▾
Why this is wrong here
A partially seated SFP would likely prevent any light from entering, not show a measurable but weak signal.
✗The single-mode fiber distance exceeds the 10 km maximum for 1000BASE-LX, leading to severe signal dispersion.Wrong answer — click to see why▾
Why this is wrong here
Distance would not cause a 27 dB loss over such a short path, and dispersion is not measured as a reduction in optical power on the DOM readout.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the concept that a link can fail due to excessive optical loss even when the fiber distance is well within the rated maximum, leading candidates to incorrectly blame distance or counterfeit hardware instead of connector cleanliness or damage.
Detailed technical explanation
How to think about this question
1000BASE-LX uses a 1310 nm laser and has a typical maximum channel insertion loss budget of about 10 dB (including connectors, splices, and fiber attenuation of ~0.4 dB/km). With a transmit power of –3 dBm and a receive sensitivity of –19 dBm, the allowable loss is 16 dB; a measured Rx power of –30 dBm represents a loss of 27 dB, far exceeding the budget. In real-world troubleshooting, a dirty connector can introduce 1–2 dB of loss per mated pair, and multiple dirty connections can quickly accumulate to exceed the link budget, even on short spans.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this 200-301 question test?
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Excessive attenuation due to a dirty or damaged fiber connector is preventing the link from coming up. — Option C is correct because the measured Rx power of –30 dBm is far below the receive sensitivity threshold for 1000BASE-LX (typically –19 to –22 dBm), even though the transmitter is outputting a healthy –3 dBm over only 2 km. This indicates excessive loss in the optical path, most commonly caused by a dirty or damaged fiber connector. Cleaning the connector ends with an appropriate fiber cleaning tool and inspecting with a microscope would likely resolve the issue.
What should I do if I get this 200-301 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.
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